Something

function user_avatar_url($mode, &$error, &$error_msg, $avatar_filename)
{
	global $lang;

	if ( !preg_match('#^(http)|(ftp):\/\/#i', $avatar_filename) )
	{
		$avatar_filename = 'http://' . $avatar_filename;
	}

	$avatar_filename = substr($avatar_filename, 0, 100);

	if ( !preg_match("#^((ht|f)tp://)([^ \?&=\#\"\n\r\t<]*?(\.(jpg|jpeg|gif|png))$)#is", $avatar_filename) )
	{
		$error = true;
		$error_msg = ( !empty($error_msg) ) ? $error_msg . '<br />' . $lang['Wrong_remote_avatar_format'] : $lang['Wrong_remote_avatar_format'];
		return;
	}

	return ( $mode == 'editprofile' ) ? ", user_avatar = '" . str_replace("\'", "''", $avatar_filename) . "', user_avatar_type = " . USER_AVATAR_REMOTE : '';

}